Structure and expression of the murine L-myc gene

Molecular Mechanisms of MYCN Dysregulation in Cancers

MYCN, a member of MYC proto-oncogene family, encodes a basic helix-loop-helix transcription factor N-MYC. Abnormal expression of N-MYC is correlated with high-risk cancers and poor prognosis. Initially identified as an amplified oncogene in neuroblastoma in 1983, the oncogenic effect of N-MYC is expanded to multiple neuronal and nonneuronal tumors. Direct targeting N-MYC remains challenge due to its "undruggable" features. Therefore, alternative therapeutic approaches for targeting MYCN-driven tumors have been focused on the disruption of transcription, translation, protein stability as well as synthetic lethality of MYCN. In this review, we summarize the latest advances in understanding the molecular mechanisms of MYCN dysregulation in cancers.

MB0 and MBI Are Independent and Distinct Transactivation Domains in MYC that Are Essential for Transformation

MYC is a transcription factor that is essential for cellular proliferation and development. Deregulation or overexpression of MYC occurs in a variety of human cancers. Ectopic expression of MYC causes hyperproliferation and transformation of cells in culture and tumorigenesis in several transgenic mouse models. Deregulation of MYC can also induce apoptosis through activation of p53 and/or ARF tumor suppressors as a safeguard to prevent tumorigenesis. MYC binds to thousands of genomic sites and regulates hundreds of target genes in a context-dependent fashion to mediate these diverse biological roles. The N-terminal region of MYC contains several conserved domains or MYC Boxes (MB), which influence the different MYC transcriptional and biological activities to varying degrees. However, the specific domains that mediate the ability of MYC to activate transcription remain ill defined. In this report, we have identified a new conserved transactivation domain (TAD), MB0, which is essential for MYC transactivation and target gene induction. We demonstrate that MB0 and MBI represent two distinct and independent TADs within the N-terminal 62 amino acids of MYC. In addition, both MB0 and MBI are essential for MYC transformation of primary fibroblasts in cooperation with activated RAS, while MB0 is necessary for efficient MYC-induced p53-independent apoptosis.

Pre-Anchoring of Pin1 to Unphosphorylated c-Myc in a Fuzzy Complex Regulates c-Myc Activity

Hierarchic phosphorylation and concomitant Pin1-mediated proline isomerization of the oncoprotein c-Myc controls its cellular stability and activity. However, the molecular basis for Pin1 recognition and catalysis of c-Myc and other multisite, disordered substrates in cell regulation and disease is unclear. By nuclear magnetic resonance, surface plasmon resonance, and molecular modeling, we show that Pin1 subdomains jointly pre-anchor unphosphorylated c-Myc1-88 in the Pin1 interdomain cleft in a disordered, or "fuzzy", complex at the herein named Myc Box 0 (MB0) conserved region N-terminal to the highly conserved Myc Box I (MBI). Ser62 phosphorylation in MBI intensifies previously transient MBI-Pin1 interactions in c-Myc1-88 binding, and increasingly engages Pin1PPIase and its catalytic region with maintained MB0 interactions. In cellular assays, MB0 mutated c-Myc shows decreased Pin1 interaction, increased protein half-life, but lowered rates of Myc-driven transcription and cell proliferation. We propose that dynamic Pin1 recognition of MB0 contributes to the regulation of c-Myc activity in cells.

Myc and its interactors take shape

The Myc oncoprotein is a key contributor to the development of many human cancers. As such, understanding its molecular activities and biological functions has been a field of active research since its discovery more than three decades ago. Genome-wide studies have revealed Myc to be a global regulator of gene expression. The identification of its DNA-binding partner protein, Max, launched an area of extensive research into both the protein-protein interactions and protein structure of Myc. In this review, we highlight key insights with respect to Myc interactors and protein structure that contribute to the understanding of Myc's roles in transcriptional regulation and cancer. Structural analyses of Myc show many critical regions with transient structures that mediate protein interactions and biological functions. Interactors, such as Max, TRRAP, and PTEF-b, provide mechanistic insight into Myc's transcriptional activities, while others, such as ubiquitin ligases, regulate the Myc protein itself. It is appreciated that Myc possesses a large interactome, yet the functional relevance of many interactors remains unknown. Here, we discuss future research trends that embrace advances in genome-wide and proteome-wide approaches to systematically elucidate mechanisms of Myc action. This article is part of a Special Issue entitled: Myc proteins in cell biology and pathology.

Mammalian MYC Proteins and Cancer

The MYC family of proteins is a group of basic-helix-loop-helix-leucine zipper transcription factors that feature prominently in cancer. Overexpression of MYC is observed in the vast majority of human malignancies and promotes an extraordinary set of changes that impact cell proliferation, growth, metabolism, DNA replication, cell cycle progression, cell adhesion, differentiation, and metastasis. The purpose of this review is to introduce the reader to the mammalian family of MYC proteins, highlight important functional properties that endow them with their potent oncogenic potential, describe their mechanisms of action and of deregulation in cancer cells, and discuss efforts to target the unique properties of MYC, and of MYC-driven tumors, to treat cancer.

L-Myc protein synthesis is initiated by internal ribosome entry

An internal ribosome entry segment (IRES) has been identified in the 5' untranslated region (5' UTR) of two members of the myc family of proto-oncogenes, c-myc and N-myc. Hence, the synthesis of c-Myc and N-Myc polypeptides can involve the alternative mechanism of internal initiation. Here, we show that the 5' UTR of L-myc, another myc family member, also contains an IRES. Previous studies have shown that the translation of mRNAs containing the c-myc and N-myc IRESs can involve both cap-dependent initiation and internal initiation. In contrast, the data presented here suggest that internal initiation can account for all of the translation initiation that occurs on an mRNA with the L-myc IRES in its 5' UTR. Like many other cellular IRESs, the L-myc IRES appears to be modular in nature and the entire 5' UTR is required for maximum IRES efficiency. The ribosome entry window within the L-myc IRES is located some distance upstream of the initiation codon, and thus, this IRES uses a "land and scan" mechanism to initiate translation. Finally, we have derived a secondary structural model for the IRES. The model confirms that the L-myc IRES is highly structured and predicts that a pseudoknot may form near the 5' end of the mRNA.

IKKalpha, IKKbeta, and NEMO/IKKgamma are each required for the NF-kappa B-mediated inflammatory response program

The IKKbeta and NEMO/IKKgamma subunits of the NF-kappaB-activating signalsome complex are known to be essential for activating NF-kappaB by inflammatory and other stress-like stimuli. However, the IKKalpha subunit is believed to be dispensable for the latter responses and instead functions as an in vivo mediator of other novel NF-kappaB-dependent and -independent functions. In contrast to this generally accepted view of IKKalpha's physiological functions, we demonstrate in mouse embryonic fibroblasts (MEFs) that, akin to IKKbeta and NEMO/IKKgamma, IKKalpha is also a global regulator of tumor necrosis factor alpha- and IL-1-responsive IKK signalsome-dependent target genes including many known NF-kappaB targets such as serum amyloid A3, C3, interleukin (IL)-6, IL-11, IL-1 receptor antagonist, vascular endothelial growth factor, Ptx3, beta(2)-microglobulin, IL-1alpha, Mcp-1 and -3, RANTES (regulated on activation normal T cell expressed and secreted), Fas antigen, Jun-B, c-Fos, macrophage colony-stimulating factor, and granulocyte-macrophage colony-stimulating factor. Only a small number of NF-kappaB-dependent target genes were preferentially dependent on IKKalpha or IKKbeta. Constitutive expression of a trans-dominant IkappaBalpha superrepressor (IkappaBalphaSR) in wild type MEFs confirmed that these signalsome-dependent target genes were also dependent on NF-kappaB. A subset of NF-kappaB target genes were IKK-dependent in the absence of exogenous stimuli, suggesting that the signalsome was also required to regulate basal levels of activated NF-kappaB in established MEFs. Overall, a sizable number of novel NF-kappaB/IKK-dependent genes were identified including Secreted Frizzled, cadherin 13, protocadherin 7, CCAAT/enhancer-binding protein-beta and -delta, osteoprotegerin, FOXC2 and FOXF2, BMP-2, p75 neurotrophin receptor, caspase-11, guanylate-binding proteins 1 and 2, ApoJ/clusterin, interferon (alpha and beta) receptor 2, decorin, osteoglycin, epiregulin, proliferins 2 and 3, stromal cell-derived factor, and cathepsins B, F, and Z. SOCS-3, a negative effector of STAT3 signaling, was found to be an NF-kappaB/IKK-induced gene, suggesting that IKK-mediated NF-kappaB activation can coordinately illicit negative effects on STAT signaling.

myc boxes, which are conserved in myc family proteins, are signals for protein degradation via the proteasome

Cellular levels of the rapidly degraded c-myc protein play an important role in determining the proliferation status of cells. Increased levels of c-myc are frequently associated with rapidly proliferating tumor cells. We show here that myc boxes I and II, found in the N termini of all members of the myc protein family, function to direct the degradation of the c-myc protein. Both myc boxes I and II contain sufficient information to independently direct the degradation of otherwise stably expressed proteins to which they are fused. At least part of the myc box-directed degradation occurs via the proteasome. The mechanism of myc box-directed degradation appears to be conserved between yeast and mammalian cells. Our results suggest that the myc boxes may play an important role in regulating the level and activity of the c-myc protein.

Expression of myc-family, myc-interacting, and myc-target genes during preimplantation mouse development

Previous studies indicated that members of the myc gene family may be essential for preimplantation development. Other studies revealed that preimplantation embryos lacking c-myc, N-myc, or L-myc are viable, indicating that these genes are either not essential for preimplantation development or can be substituted for functionally by other myc gene family members. To investigate the possible role of these genes during preimplantation development, we determined the temporal patterns of expression of four members of the myc gene family, genes encoding myc-associated proteins, and four putative MYC target genes. We observed a sequential pattern of myc gene expression, with the L-myc mRNA expressed as a maternal transcript, the c-myc mRNA expressed during the 4-cell through morula stages, and the B-myc mRNA expressed highly at the morula and blastocysts stages. B-myc was the predominant family member expressed during preimplantation development. The mxi mRNA was not detectable and the mad mRNA was detectable only as a maternal transcript. The max mRNA, however, was expressed both as a maternal mRNA and as an embryonic message throughout most of preimplantation development. Three putative MYC target genes (Odc, cyclin E, and prothymosin-alpha) were transcriptionally induced during the 2-cell stage, and their mRNAs increased sharply in abundance during development to the morula and blastocyst stages. Another putative MYC target gene, cyclin A, was expressed both as a maternal mRNA and as an embryonic transcript. These data support the view that the expression of myc target genes may be supported initially through the expression of maternally inherited MYC proteins and corresponding mRNAs and that subsequent stage-specific changes in expression of myc genes, myc-associated genes, and myc target genes may control early differentiative events around the time of implantation.

The amplification of oligonucleotide themes in the evolution of the myc protooncogene family

The evolutionary past of intragenic repeats in protein-coding exons of c-, N-, L-, and s-myc-protooncogene subfamilies was elucidated. Apparently these genes evolved by succession of distinct unit events rather than by a steady flow of random point mutations. An evolutionary event probably involved a duplication of the whole gene, which was followed by amplification of progressively shorter oligonucleotide themes and motifs. The repeats were either joined in tandem or one of the copies was transposed and integrated elsewhere within the same exon. In some instances multiple fragments of an amplified theme were integrated at several sites. Direct repeats were found to prevail over inverted ones. By reconstructing the fate of repeats in the course of evolution of vertebrates, the origins of some functional domains could be traced to the initial amplification event. For example, an N-myc-specific domain was created by tandem duplication of a single-copy theme of L-myc exon; at the time of divergence of the c-myc and N-myc, the tandem duplex underwent a new round of duplication followed by transposition of the new copy, thus accounting for the formation of a new domain specific for c-myc. The model proposed here may be regarded as a molecular-level equivalent of the theory of punctuated equilibria.

Heritable formation of neuroectodermal tumor in transgenic mice carrying the combined E1 region gene of adenovirus type 12 with the deregulated human renin promoter

Adenovirus early 1 (E1) region gene products, including E1A and E1B, are required for transcriptional regulation of viral and cellular promoters in infected and transfected culture cells and for transformation of primary rodent cells. Here, we established a line of transgenic mice carrying the E1 region gene of human adenovirus type 12 under the control of the human renin promoter, in which a neuroectodermal tumor derived from retroperitoneal, olfactory, and/or pelvic regions was heritably developed with varying degrees of incidence and the phenotype was successfully passed through six generations. The transgenes were located in the region E2-E3 bands of chromosome 7 with which no genetic linkage to neuroectodermal tumors was previously demonstrated, and expressed only in the tumors but not in another tissue examined. Notably, in addition to the expression of a neural marker gene N-CAM, the three nuclear oncogenes, c-, L-, and N-myc, were coexpressed in the tumors. These results suggest that E1A and E1B are cooperatively involved in the heritable formation of neuroectodermal tumors associated with co-expression of the three sets of myc family genes.

Point mutations in the c-Myc transactivation domain are common in Burkitt's lymphoma and mouse plasmacytomas

We have screened the entire coding region of c-myc in a panel of Burkitt's lymphomas (BLs) and mouse plasmacytomas (PCTs). Contrary to the belief that c-myc is wild type in these tumours, we found that 65% of 57 BLs and 30% of 10 PCTs tested exhibit at least one amino acid (aa) substitution. These mutations were apparently homozygous in all BL cell lines tested and two tumour biopsies, implying that the mutations often occur before Myc/Ig translocation in BL. In PCTs, only the mutant c-myc allele was expressed indicating a functional homozygosity, but occurrence of mutations after the translocation. Many of the observed mutations are clustered in regions associated with transcriptional activation and apoptosis, and in BLs, they frequently occur at sites of phosphorylation, suggesting that the mutations have a pathogenetic role.

Phylogenetic relationships among laboratory and wild-origin Mus musculus strains on the basis of genomic DNA RFLPs

Genetic distance measures between the laboratory mouse strains C57BL/6J and RF/J and the wild-origin Mus musculus mouse strains CAST/Ei, MOLF/Ei, POSCH I, and CZECH II were estimated by allelic patterns revealed by RFLP analysis. These results suggest phylogenetic relationships indicating that the mouse strains related to the subspecies M.m. domesticus (RF/J, POSCH I and C57BL/6J) are more closely related to the CAST/Ei strain (derived from M.m. castaneus) than to the strains CZECH II (M.m. musculus) and MOLF/Ei (M.m. molossinus). Furthermore, the hybrid strain C57BL/6J is more closely related to POSCH I (M.m. poschiavinus) than to RF/J as calculated by the method distance measures of Cavalli-Sforza and Edwards (Evolution 21,550, 1967), Nei's minimum (Am. Natural. 106,283, 1972) and unbiased minimum (Genetics 89,583, 1978), Edwards (Biometrics 27,873, 1971; Genetic Distance, p. 41, 1974) and Rogers modified (1986).

Changes of DNA methylation in protooncogenes in the process of radiation-induced transformation of mouse m5S/1M cells in vitro

To assess the importance of changes in DNA methylation in an X-ray-induced cellular transformation process, methylation patterns of five nuclear protooncogenes in fifteen transformant clones were studied and compared to that of the parental non-transformed cell line m5S/1M. All transformants examined revealed an alteration in DNA methylation in some of the genes, although these changes were variable among them. A comparison of cellular characteristics with corresponding DNA methylation changes in different clones suggested that the loss of contact inhibition and the gain of anchorage independency were associated with increases of methylation in many genes, whereas the acquisition of tumorigenicity was often accompanied by a decrease of methylation in the N-myc and c-myc genes. Resultant data indicate that the alteration of DNA methylation is closely related to transformation process, yet how this involvement occurs is complex and remains unclear.

L-myc and N-myc influence lineage determination in the central nervous system

The N-myc and the L-myc proto-oncogenes are expressed during embryonal development mainly in the developing brain. Studies of their expression in single neuroepithelial cells revealed that neural precursors not yet committed to the glial or the neuronal lineage expressed both genes, but after lineage commitment they expressed either N-myc or L-myc. Moreover, enforced expression of L-myc in the neural precursor cell line 2.3D caused neuronal differentiation, while the expression of N-myc promoted glial differentiation. These results indicate that L-myc and N-myc play critical roles in lineage determination for the central nervous system.

Myc family oncoproteins function through a common pathway to transform normal cells in culture: cross-interference by Max and trans-acting dominant mutants

The myc family of cellular oncogenes encodes three highly related nuclear phosphoproteins (c-Myc, N-Myc, and L-Myc) that are believed to function as sequence-specific transcription factors capable of regulating genes important in cellular growth and differentiation. Current evidence indicates that Myc family proteins exist as biologically active heterodimeric complexes in association with another helix-loop-helix leucine zipper phosphoprotein, Max. We have investigated the common and unique properties among the Myc family, as well as the physiological role of Max in the regulation of Myc family function. We demonstrate that trans-activation-incompetent mutants of one Myc family member can act in trans to dominantly suppress the cotransformation activities of all three Myc oncoproteins, indicating that the Myc family functions through common genetic elements in its cellular transformation pathways. Employing co-immunoprecipitation with either anti-Myc or anti-Max antibodies, we show that the transfected normal c-Myc, N-Myc, and L-Myc oncoproteins associate with the endogenous Max protein in REF transformants, indicating that the Max interaction represents at least one component common to Myc family function. In addition, we observed a striking reduction in Myc cotransformation activity when a Max expression construct was added to myc/ras co-transfections. We discuss these biological findings in the context of a proposed model for Myc/Max function and regulation in which Max serves as either an obligate partner in the Myc/Max transcriptional complex or as a repressor in the form of a transcriptionally inert Max/Max homodimer capable of occupying Myc/Max-responsive gene targets.

Molecular genetic linkage maps of mouse chromosomes 4 and 6

We have generated a moderate resolution genetic map of mouse chromosomes 4 and 6 utilizing a (C57BL/6J x Mus spretus) F1 x Mus spretus backcross with RFLPs for 31 probes. The map for chromosome 4 covers 77 cM and details a large region of homology to human chromosome 1p. The map establishes the breakpoints in the mouse 4-human 1p region of homology to a 2-cM interval between Ifa and Jun in mouse and to the interval between JUN and ACADM in human. The map for mouse chromosome 6 spans a 65-cM region and contains a large region of homology to human 7q. These maps also provide chromosomal assignment and order for a number of previously unmapped probes. The maps should allow the rapid regional assignment of new markers to mouse chromosomes 4 and 6. In addition, knowledge of the gene order in mouse may prove useful in determining the gene order of the homologous regions in human.

Nef proteins of the human immunodeficiency viruses (HIV-1 and HIV-2) and simian immunodeficiency virus (SIV) are structurally similar to leucine zipper transcriptional activation factors

Analysis of the predicted amino acid sequences of the human immunodeficiency virus types 1 and 2 (HIV-1 and HIV-2) and of the related simian immunodeficiency virus (SIV) nef gene products (Nef) reveals the presence of a conserved leucine zipper-like repeat with the characteristic 4,3 arrangement of mainly hydrophobic amino acids in the middle (core) region of the proteins, but lacking the basic (DNA binding) domain characteristic of DNA-binding leucine zipper (bZIP) proteins. Also, at the C-terminus of the Nef proteins is a highly acidic sequence (net charge of -5 to -8) stretched over about 40 amino acids, and contains two predicted alpha-helices separated by a beta-turn linker sequence with sequence homology to known activation domains of acidic transcriptional activation factors. Moreover, within this acidic region of transcriptional activators and the homologous sequence within the second Nef alpha-helix, is a potential transcriptional activation consensus sequence (TACS) bounded by a pair of acidic amino acids (aspartic or glutamic acids) at the N-terminus and a highly invariant phenylalanine (hydrophobic), often followed by an acidic (aspartic) residue, at the C-terminus of the sequence. These findings strongly implicate Nef proteins as belonging to a class of non-DNA-binding leucine zipper acidic transcription factors, and provide a structural basis for new approaches to studying Nef function.

Genetic mapping of meander tail, a mouse mutation affecting cerebellar development

The meander tail mouse harbors a recessive mutation on chromosome 4 that affects the anterior lobes of the cerebellum and the caudal vertebrae. Examination of the mea/mea cerebellum reveals that the complete disorganization of all cell types seen in the anterior lobes is separated by a sharp and consistent boundary from the normal cytoarchitecture of the posterior lobes. In the absence of any biochemical information regarding the affected gene product, attempts to clone the gene must rely on the strategy of reverse genetics. As an initial step in this process we have constructed a genetic linkage map spanning 68 cM of chromosome 4 using an intersubspecific phenotypic backcross. The loci included in this analysis are Calb, Ggtb, Lv, b, Ifa, mea, D4Rp1, Glut-1, Lck, Lmyc-1, and Eno-1. This analysis positions the mea phenotypic locus in the interval between Ifa and Glut1. These results also further define regions of homology between mouse chromosome 4 and human chromosomes 8, 1, and 9. This linkage map provides the means to evaluate candidate genes, and to identify tightly linked markers useful for cloning the meander tail locus.

Provirus integration at the 3' region of N-myc in cell lines established from thymic lymphomas spontaneously formed in AKR mice and a [(BALB/c x B6)F1----AKR] bone marrow chimera

Among 18 thymic leukemia cell lines which have been established from spontaneous thymic lymphomas in AKR mice as well as in bone marrow chimeras which were constructed by transplanting allogeneic bone marrow cells into irradiated AKR mice, three proviral integration sites were identified; near c-myc, N-myc and pim-1 loci. No integration site specific for chimeric leukemia cell lines was found. In three thymic leukemia cell lines which contained rearranged N-myc genes, insertions of long terminal repeats (LTRs) of murine leukemia viruses were detected at 18 or 20 bp downstream of the translational termination codon. These results demonstrate that the 3' region of the N-myc gene is one of the integration targets for murine leukemia viruses in spontaneous thymic lymphomas. In these three cell lines, N-myc mRNA was stably transcribed and transcription of c-myc mRNA was down-regulated. The integrated murine leukemia viruses in AKR thymic leukemia were most likely AKV, though the DNA sequence of the LTR inserted in the genome of a leukemic cell line from [(BALB/c x B6)F1----AKR], CAK20, was different from LTRs of murine leukemia viruses so far reported.

DNA methylation and oncogene expression in methapyrilene-induced rat liver tumors and in treated hepatocytes in culture

Continued exposure of rats to carcinogenic doses of methapyrilene (MP) leads to elevated levels of 5-methyl-deoxycytidine (5MC) in liver DNA. Since gene expression often correlates with DNA methylation, we investigated these parameters in the MP-induced hepatocellular carcinomas of Fischer 344 rats. DNA was hypermethylated in liver tissue surrounding the tumors relative to liver tissue of untreated controls of the same age, while tumor DNA was not; DNA methylation declined to normal levels when MP treatment ceased. Gene expression analysis showed measurable levels of mRNA for c-Ki-ras, erb-B, erb-B2, hck, src, lyn, vav, trk, raf-1, l-myc, c-jun, c-yes, c-myc, c-abl, and p53. No significant differences in expression for these and other oncogenes were seen between tumors and surrounding livers, although erb-B2 and vav showed visible decreases compared with normal liver. Hypermethylation of DNA and expression of these oncogenes in MP-treated tissues were not correlated. Levels of mRNA for the same genes in MP-treated hepatocytes in culture were similar to in vivo levels; analysis of DNA synthesis levels showed that this gene expression pattern occurred in the absence of proliferation bursts or toxicity in these cells, thus suggesting that treatment in vivo may produce the same results.

Molecular analysis of a cellular decision during embryonic development of Drosophila melanogaster: epidermogenesis or neurogenesis

In Drosophila melanogaster, the neuroblasts (neural progenitor cells) develop from a special region of the ectoderm, called the neuroectoderm. During early embryonic development, the neuroblasts separate from the remaining cells of the neuroectoderm, which develop as epidermoblasts (epidermal progenitor cells). The separation of these two cell types is the result of cellular interactions. The available data indicate that a signal chain formed by the products of several identified genes regulates the cell's decision to enter either neurogenesis or epidermogenesis. Various kinds of data, in particular from cell transplantation studies and from genetic and molecular analyses, suggest that the proteins encoded by the genes Notch and Delta interact at the membrane of the neuroectodermal cells to provide a regulatory signal. This signal is thought to lead, on the one hand, to epidermal development through the action of the genes of the Enhancer of split complex, a gene complex that encodes several functions related to the transduction and further processing of the signal, including the genetic regulation in the receiving cell; on the other hand, the signal is thought to lead to neural development through the participation of the genes of the achaete-scute complex and daughterless, which are members of a family of DNA-binding regulatory proteins and of the gene vnd whose molecular nature is still unknown.

The protein Id: a negative regulator of helix-loop-helix DNA binding proteins

We have isolated a cDNA clone encoding a novel helix-loop-helix (HLH) protein, Id. Id is missing the basic region adjacent to the HLH domain that is essential for specific DNA binding in another HLH protein, MyoD. An in vitro translation product of Id can associate specifically with at least three HLH proteins (MyoD, E12, and E47) and attenuate their ability to bind DNA as homodimeric or heterodimeric complexes. Id is expressed at varying levels in all cell lines tested. In three cell lines that can be induced to undergo terminal differentiation, Id RNA levels decrease upon induction. Transfection experiments indicate that over-expression of Id inhibits the trans-activation of the muscle creatine kinase enhancer by MyoD. Based on these findings, we propose that HLH proteins lacking a basic region may negatively regulate other HLH proteins through the formation of nonfunctional heterodimeric complexes.

Domains of the SFL1 protein of yeasts are homologous to Myc oncoproteins or yeast heat-shock transcription factor

We identified a yeast suppressor gene for flocculation (SFL1), which complemented a newly isolated sfl1 mutant. This mutation causes asexual cell aggregation. SFL1 encodes a 767-amino acid protein which has two domains significantly homologous to Myc oncoproteins and the yeast heat shock transcription factor (HSTF). The Myc homologous region in SFL1 overlaps with the conserved region in a series of interesting proteins: MyoD1, Drosophila achaete-scute, twist, daughterless gene products and immunoglobulin enhancer-binding proteins. In addition, the N-terminal region of the SFL1 gene product shows extensive homology to the DNA-binding domain of HSTF. Mutational analysis of SFL1 demonstrates that it is required for normal cell-surface assembly in vegetative growth. We propose that the SFL1 gene product may be a transcription factor which is involved in regulation of the gene(s) related to yeast flocculation.

A molecular genetic linkage map of mouse chromosome 4 including the localization of several proto-oncogenes

We have constructed a 64-cM molecular genetic linkage map of mouse chromosome 4 using interspecific backcross animals derived from mating C57BL/6J and Mus spretus mice. Several proto-oncogenes and common sites of viral integration have been assigned regional locations on chromosome 4 including Mos, Lyn, Jun, Lmyc, Lck, Fgr, and Dsi-1. Additional loci mapped in this study to chromosome 4 were Tsha, Mup-1, Rrm2-ps1, Ifa, and Anf. A comparison of our mapping data with inbred strain mapping data did not show any evidence for inversions or deletions on chromosome 4. New regions of synteny were defined between mouse chromosome 4 and human chromosomes 1 and 8; a region of homology was found between mouse chromosome 4 and human chromosome 6. This linkage map will provide a framework for identifying homologous genes in mice and humans that may be involved in various disease processes.

The gene for enhancer binding proteins E12/E47 lies at the t(1;19) breakpoint in acute leukemias

The gene (E2A) that codes for proteins with the properties of immunoglobulin enhancer binding factors E12/E47 was mapped to chromosome region 19p13.2-p13.3, a site associated with nonrandom translocations in acute lymphoblastic leukemias. The majority of t(1;19)(q23;p13)-carrying leukemias and cell lines studied contained rearrangements of E2A as determined by DNA blot analyses. The rearrangements altered the E2A transcriptional unit, resulting in the synthesis of a transcript larger than the normal-sized E2A mRNAs in one of the cell lines with this translocation. These observations indicate that the gene for a transcription factor is located at the breakpoint of a consistently recurring chromosomal translocation in many acute leukemias and suggest a direct role for alteration of such factors in the pathogenesis of some malignancies.

Role of the c-myc and the N-myc proto-oncogenes in the immortalization of neural precursors

To study the role of c-myc and N-myc in the immortalization of neural precursors, we infected neuroepithelial cells isolated from 10-day-old mouse embryos with a new retrovirus vector, pzen, harboring either the c-myc or the N-myc oncogene. The immortalized cell lines contain high levels of the virally expressed myc protein. The amount of myc proteins correlated with the capacity of the cells to differentiate spontaneously in vitro into neurons and glia; cell lines expressing high levels of myc protein can differentiate spontaneously while cell lines expressing low levels of the myc protein resemble epithelial cells. Addition of acidic or basic fibroblast growth factor enhanced differentiation of most cell lines. Some of the cell lines produced neurotrophic growth factors capable of supporting the growth of other cell lines at low density. There was no significant difference between cell lines immortalized with c-myc or with N-myc. Most of the immortalized cells lines generated from bipotential precursors are capable of differentiating into neurons and glia.

lyl-1, a novel gene altered by chromosomal translocation in T cell leukemia, codes for a protein with a helix-loop-helix DNA binding motif

We have characterized a transcription unit at chromosome band 19p13 that lies at the site of a chromosomal translocation breakpoint in T cell acute lymphoblastic leukemia. The lyl-1 gene is structurally altered following a t(7;19) translocation, resulting in its head-to-head juxtaposition with the T cell receptor C beta gene and truncation of lyl-1 RNA. The predicted protein product of the lyl-1 gene contains a potential helix-loop-helix DNA binding motif also found in several proteins involved in the control of cellular proliferation and differentiation: all members of the Myc family, MyoD1, myogenin, the Drosophila achaete-scute, twist, and daughterless proteins, and two recently described immunoglobulin enhancer binding proteins. The implication of lyl-1 in cellular transformation suggests that other proteins containing similar DNA binding motifs may also be involved with neoplastic transformation in various cellular lineages.

Genetic alterations in primary breast cancer

A serious effort has been made to identify and characterize mutations that frequently occur during the evolution of primary human breast cancer. Some of these mutations involve amplification of protooncogenes (c-myc, c-erbB-2, and int-2) that have been shown to contribute to experimentally induced breast cancer in mouse model systems. Tumor development in mice containing the c-myc or c-erbB-2 transgene suggests that the cellular and developmental contexts in which the genes are expressed define their relative contribution to tumorigenesis. Homozygous deletions or loss of heterozygosity (LOH) represent another type of mutation that has been frequently observed on four chromosomes (1q, 3p, 11p, and 13q) in tumor DNA. They are thought to unmask recessive mutations (LOH) that inactivate or remove (homozygous deletion) suppressor genes that regulate normal cell proliferation. Attempts to determine whether specific mutations are associated with certain clinical parameters have led to the controversial hypothesis that some mutations may be useful prognostic indicators of the post-surgical course of the disease. The current results underscore the necessity for much larger, better control studies to unambiguously define the potential of such mutations as clinical markers.

myc-related proteins and DNA sequences in Trypanosoma brucei

The cAMP content of Trypanosoma brucei increases in parallel with ascending mammalian parasitemia to very high levels just before differentiation of the long-slender to the short-stumpy bloodstream form. Because expression of myc oncogenes is required for vertebrate cells to interpret proliferation signals and declines in response to cAMP mediated differentiation, we investigated whether T. brucei also harbored myc-like proteins and genes. Accordingly, we probed lysates of long-slenders, short-stumpies and procyclics (insect midgut stage) with antibody to myc proteins and also hybridized myc gene family sequences to procyclic DNA. We found that antibody to myc-family proteins of mammals reacts with 40 kDa and 55 kDa proteins in all three life cycle stages, and that procyclic DNA contains three EcoRI fragments that are homologous to a v-myc probe. One of these fragments also hybridizes to a synthetic 25-mer oligonucleotide deduced from a consensus sequence in the second exon of the myc family and expresses a 3.2 kb mRNA transcript in Northern blots of procyclic RNA. The conservation of myc-family homologous across the broad phylogenetic gap between mammals and trypanosomes illustrates ancient evolutionary relationships and raises the possibility of stage-specific expression of myc genes during the life cycle of T. brucei.

SPXX, a frequent sequence motif in gene regulatory proteins

A new DNA-binding unit, composed of four amino acid residues and common in gene regulatory proteins, is proposed. The occurrences of the sequences Ser-Pro-X-X (SPXX) and Thr-Pro-X-X (TPXX) in gene regulatory proteins are compared with those in general proteins. These sequences are found more frequently in gene regulatory proteins including homoeotic gene products, segmentation gene products, steroid hormone receptors and certain oncogene products, than they are in DNA-binding proteins that are not directly involved in gene regulation, such as the core histones, or in general proteins. It is therefore suggested that these sequences contribute to DNA-binding in a manner important for gene regulation. Amino acid residues characteristic of the types of proteins are found as the variable residues X: basic residues, Lys and Arg, in histones, H1 and sea urchin spermatogenous H2B; Tyr in RNA polymerase II; and Ser, Thr, Ala, Leu and Pro in other gene regulatory proteins S(T)PXX sequences are located on either side of other DNA-recognizing units such as Zn fingers, helix-turn-helices, and cores of histones. The structure of a S(T)PXX sequence is presumed to be a beta-turn I stabilized by two hydrogen bonds, and its potential mode of DNA-binding is discussed.

Mapping and characterization of an X-linked processed gene related to MYCL1

A DNA sequence with homology to the myc family of proto-oncogenes has been characterized and found to be a processed gene related to L-MYC (MYCL1). This processed gene (MYCL2) was isolated by cross-hybridization to an oligonucleotide probe synthesized from the C-MYC (MYC) sequence in a highly conserved region of the myc gene family. Sequence analysis of MYCL2 revealed an open reading frame of 1194 bp with no intervening sequences and strong homology to the recently published DNA sequence of MYCL1. Southern and Northern blot analyses of DNAs and RNAs from small cell lung carcinomas confirmed its MYCL1 homology. Mapping of MYCL2 by somatic cell hybrids places this sequence on the long arm of the X chromosome in bands q22----q28.

The myc family of nuclear proto-oncogenes

Several members of the myc family of proto-oncogenes have been described, and some (c-, N-, and L-myc) have been characterized in considerable detail. They are united by a common gene structure and nucleotide homologies that were used to identify some of them initially. Their protein products also have scattered regions of amino acid identity or homology. Although the cellular activities of the various proteins are unknown, some members may play a role in regulating cell growth and differentiation. They share the ability to cooperate with an activated ras gene and cotransform embryonic rodent cells. In naturally occurring tumors, the members of the myc family of oncogenes appear to be activated by genetic changes (proviral insertion, chromosomal translocation, and gene amplification) that augment or otherwise disrupt normally regulated expression. The members of this family of genes differ markedly in their tissue specificity and developmental regulation of expression. This may account in part for the frequent appearance of activated c-myc genes in a wide variety of neoplasms and the limited appearance of activated N- and L-myc genes in tumors of embryonic or neural origin. The c-myc gene may be activated in tumors by a variety of mechanisms, whereas N- and L-myc appear to be activated only by gene amplification. Regulation of expression of the different myc genes also appears to occur by different mechanisms. Finally, the products of the different genes differ in may regions of the protein, and this divergence probably reflects their specific and individual functions.

The role of the leucine zipper in the fos-jun interaction

Mutagenesis of the fos protein supports the hypothesis that a heptad repeat of leucine residues stabilizes the interaction between the fos and jun proteins. We show that the complex between fos and jun can bind to DNA more tightly than either protein alone and that basic residues adjacent to the leucine repeat of fos contribute to the DNA-binding potential of the complex.

The human myc gene family: structure and activity of L-myc and an L-myc pseudogene

We have determined the nucleotide sequence and transforming activity of the human L-myc gene and a processed L-myc pseudogene (L-myc psi). We demonstrate by cotransformation assays that a 10.6-kb EcoRI fragment derived from a human placental library contains a complete and functional L-myc gene including transcriptional regulatory sequences sufficient for expression in rat embryo fibroblasts. Organization of the L-myc gene was determined by comparing its sequence to those of the L-myc psi gene and an L-myc cDNA clone derived from a human small cell lung carcinoma. Our results show that L-myc has a three-exon organization similar to that of the c-myc and N-myc genes. The putative L-myc gene product consists of 364 amino acids and contains five of the seven homology regions highly conserved between c-myc and N-myc. These conserved regions are located along the entire length of the putative L-myc protein and are interspersed among nonconserved regions. While the putative L-myc gene product is of a smaller size when compared to the c- and N-myc proteins, the relative positions of certain conserved residues occur in corresponding locations along the peptide backbone of the three proteins. In addition, comparison of the human and murine L-myc gene sequences indicate that the relatively large 5' and 3' untranslated regions are evolutionarily conserved, but that these sequences are totally divergent between the L-, c-, and N-myc genes. Finally, we demonstrate that, like the N- and c-myc genes, the L-myc gene can cooperate with a mutant Ha-ras gene to cause malignant transformation of rat embryo fibroblasts in culture. Our analyses clearly prove that L-myc represents a functional member of the myc oncogene family and further delineate structural features that may be important for the common and divergent functions of the members of this gene family.